Synonyms for Billboard:

n.

banner, bumper sticker, classified ad, circular, banner ad, advertorial, advert. placard, poster, sign, bill, show, notice. artifact (noun)
hoarding.
billboard (noun)
hoarding.
sign (noun)
poster, placard, hoarding, advertisement.
signboard (noun)
announcement, display panel, advertisement, poster board, bulletin board.

Other synonyms:

advertorial, bumper sticker, classified ad, banner, banner ad, advertisement, advert. circular, placard. notice. sign. bill. placard
poster.
Other relevant words:
announcement, advert, hoarding, bulletin board, circular, classified ad, banner, sign, notice, poster, advertorial, bill, placard, advertisement.